Re: Admission Control

On 10/07/10 03:54, Kevin Grittner wrote:
> Mark Kirkwood<mark.kirkwood@catalyst.net.nz>  wrote:
>
>    
>> Purely out of interest, since the old repo is still there, I had a
>> quick look at measuring the overhead, using 8.4's pgbench to run
>> two custom scripts: one consisting of a single 'SELECT 1', the
>> other having 100 'SELECT 1' - the latter being probably the worst
>> case scenario. Running 1,2,4,8 clients and 1000-10000 transactions
>> gives an overhead in the 5-8% range [1] (i.e transactions/s
>> decrease by this amount with the scheduler turned on [2]). While a
>> lot better than 30% (!) it is certainly higher than we'd like.
>>      
>
> Hmmm...  In my first benchmarks of the serializable patch I was
> likewise stressing a RAM-only run to see how much overhead was added
> to a very small database transaction, and wound up with about 8%.
> By profiling where the time was going with and without the patch,
> I narrowed it down to lock contention.  Reworking my LW locking
> strategy brought it down to 1.8%.  I'd bet there's room for similar
> improvement in the "active transaction" limit you describe. In fact,
> if you could bring the code inside blocks of code already covered by
> locks, I would think you could get it down to where it would be hard
> to find in the noise.
>
>    

Yeah, excellent suggestion - I suspect there is room for considerable 
optimization along the lines you suggest... at the time the focus was 
heavily biased toward a purely DW workload where the overhead vanished 
against large plan and execute times, but this could be revisited. 
Having said that I suspect a re-architect is needed for a more 
wideranging solution suitable for Postgres (as opposed to Bizgres or 
Greenplum)

Cheers

Mark